It started with the sound of a giant crack, like the Earth was coming apart.

That’s because it was.

In the next 10 seconds, the roar of a thundering landslide filled the air as a natural rock archway on the Marin coast buckled and collapsed.

The event was captured in these two photo galleries by Robert Wills: above, a wide view of the site, before-and-after; below, a sequence that shows the collapse.

The first two photos were taken about two minutes before the ultimate collapse. The sequence that follows is second-by-second over a 10-second span. If you have high-speed internet service and click on the photo arrow quickly, you can make the slide show look like a movie of the event:

The landslide took place adjacent to Tennessee Beach at the beautiful cove and pay-off for the easy hike through Tennessee Valley in the Marin Headlands. It is one of the most popular easy hikes in the Bay Area.

Robert Wills, a graduate student at the California Institute of Technology, was on the beach with his family, including his father, Chris Wills of the California Geological Survey. So they had the smarts to know what they were seeing. At the first sound of the collapse, they turned and started clicking off photos. Even for scientists, it is a once-in-a-lifetime event.

Robert Wills provided the following narrative:

“I witnessed a spectacular event. I was on a short hike with my family to Tennessee Beach, just north of the Golden Gate Bridge in San Francisco. We had come to see the large waves forecasted for the day associated with a passing storm and strong winds off the coast. The waves were indeed spectacular with 10-foot swells breaking directly onto the sand and even larger waves visible all the way to the horizon, breaking on a shoal offshore. But we were in for a far grander spectacle.

“As we were admiring the waves, a crack and the ensuing sound of a waterfall of rocks caught our attention. Everyone on the beach spun to see a small stream of rock flowing down the cliff face below the arch. I never could have expected what happened next. Two minutes later, a few chunks of rock fell from the underside of the arch and I started my camera.

“Then the arch started to buckle in on itself and squeeze out rock from beneath it. With this, the collapse of the arch shortly followed and the entire surrounding hillside started to slip off into the ocean in a thundering roar. Boulders the size of a piano crashed into the surf and the sand, sending up a large splash of debris that got me a little nervous. It all lasted less than 10 seconds and left the beach quiet in comparison. T he roar of the surf nothing compared to the thunder of the rockslide.

The sequence of photos from the main collapse provide a rare glimpse into the mechanics of the powerful, erosive force of nature. Because these events are completely unpredictable, there are few examples of such images (some of the best available landslide videos can be found on the Landslide Blog).

These images show how once the “keystone,” the rock in the center of the arch, is gone, the arch can no longer hold its own weight and collapses in on itself.

While rockfalls like this are not often seen, the presence of vertical cliffs near the ocean tell us that they are happening all the time. At most beaches you can even see sections of the cliff that look like fresh scars. So next time you are at the beach, look around and see how the cliffs have gradually fallen into the sea and don’t always trust that that cliff is going to stay there forever.